| bluebeard |
What is the simplest proposition for achieving 'fan on demand' with a thermistor?
I've got a handful of likely thermistors I can use. It's a noncritical application.. just turn on a 120vac muffin at about 100F and off again when it gets back to room temp.
I looked about for kits/boards and came up pretty empty. Is there a really simple circuit posted somewhere?
thanks |
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| Christer |
| The simplest is to use one of those thermoswitches (whatever they are called). It is a switch in a metal case which is designed open (or close) at a certain temperature. You can mount it on the heatsink and use it to switch the fan on and off. Of course, it gives no regulation, there is just on or off. |
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| wakibaki |
A simple thermostat...
Make your thermistor one of R3, R4, and make R3=R4. Any single-rail opamp will do, make the supply voltage = relay voltage.
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| wakibaki |
If you put a positive feedback resistor or preferably a pot from the output (1) of the opamp to the noninverting input (3),you will get some hysteresis, i.e. separate on and off points.
A signal diode (1N4148) may be adequate if the relay is small, in fact you might get away without it, but if the coil is big you might need a power Shottky. The deterioration in performance of the opamp due to reverse voltage when the coil is switched off may not manifest itself immediately...
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| wakibaki |
With hysteresis...
The feedback is just a starting value. R3/R4 may have to swap, depending on the tempco.
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| I_Forgot |
Instead of a thermistor that reacts to an already elevated temperature, you can monitor the load current and use it to trigger the fan. Since it takes a while for heat to build once the high current condition occurs, triggering the fan based on the current will reduce the maximum temperature rise.
I_F |
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| bluebeard |
Damn,
Yer gonna hate me for this after all the excellent suggestions. I may use some of them on something in the future. But this is really a simpler need.
I've got a 1940s Ambassador record changer that is a bit of a treasure. All in one little lift-lid box with walnut veneer and little tube amp. Everything is completely original. It was hardly ever used. I appreciate it especially for spinning ancient 78s. There's just something heart warming about this music on this player on a pedestal table in the corner of the room sometimes.
But it tends to rapidly overheat. The tubes and old motor with less than adequate ventilation just build up pretty fast. I don't want to do any real mods to this. But I'd like to put a little fan inside to boost movement out of the vents. The added fan noise and possible mechanical noise is not much concern at this level of fidelity.
So I think I'll just put a five minute delay on break relay before the fan and call it good. That'll make the ol girl happy. |
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| I_Forgot |
Why bother with a delay? Just run the fan whenever the power is on.
I_F |
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| bluebeard |
Mostly because I'm compelled in most cases to making my life difficult (as recent threads would attest).
But i'll give in. I'd like it to be cooling the motor (a quite bulky, heavily laminated job which stores tons of heat) between frequent cycles. There is no 'main' on/off switch. The knob which initiates the platter/changer cycle turns the motor and amp off each time a record or a stack finishes. I could add an on/off switch with a little indicator lamp. But that's a bit of a major modification.
I'll just go ahead and run the fan when the motor is running. Originally I was hoping that a quick two or three element circuit with thermistor might be applicable to just adhering the thermistor to the motor laminations and it would be a neat little soulution. I have so many projects laying about right now waiting for exactly the right fix that I can't afford (for sanitys' sake) to add this to the pile!
Thanks again for the suggestions.
In fact christers' suggestion might be in the offing next time I order a box of parts from digikey. They have a little canned thermoswitch that operates at 40C for about six bucks. |
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| Nordic |
How does one use the NC (Normaly Closed) ones?
I understand to use a normaly open one is easy... you just wire it all up and wait for the temperature to close the circuit...
But I can't get my head around the Normaly Open ones... which are the common types in the temperature ranges usefull for an amplifier... |
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| bluebeard |
I don't know. I'd guess that the normally closed would apply to a household appliance (where these are commonly implemented) where the switch is controlling an electric heating element or a gas pilot and solenoid. Heats up to a point and then opens/switches off the supply of heat to whatever is going on.
Since we're usually concerned with removing instead of applying heat, a normally open would be most often used with a fan arrangement. |
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| Nordic |
| Thanks bluebeard, I didn't think of the cases where you don't want to go below a temp. So it seems NC ones are not suitable then for our job. |
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| Zen Mod |
| quote: | Originally posted by Nordic
How does one use the NC (Normaly Closed) ones?
I understand to use a normaly open one is easy... you just wire it all up and wait for the temperature to close the circuit...
But I can't get my head around the Normaly Open ones... which are the common types in the temperature ranges usefull for an amplifier... |
if you have one with adequate temp , just use relay with it ; |
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| AndrewT |
| quote: | Originally posted by bluebeard
....I've got a 1940s Ambassador record changer that is a bit of a treasure. All in one little lift-lid box with walnut veneer and little tube amp. Everything is completely original. It was hardly ever used. I appreciate it especially for spinning ancient 78s. There's just something heart warming about this music on this player on a pedestal table in the corner of the room sometimes.
But it tends to rapidly overheat. |
it's around 60 years old and still works.
It is not overheating.
It just runs hotter than you expect having been exposed to cooler running solid state electronics for the last 40years. |
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| Christer |
| quote: | Originally posted by Nordic
Thanks bluebeard, I didn't think of the cases where you don't want to go below a temp. So it seems NC ones are not suitable then for our job. |
Not all components are designed for audio use, you know. ;) |
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| bluebeard |
| It's not seriously overheating (no smoke anyway), but the primary cause of breakdown with this older stuff was probably caused by heat.. and I'm not looking forward to trying to replace what's in there when/if it fails. They didn't have the ultraquiet low vibration muffin fans we have today. Thought it'd be a reasonable idea to add a little modern safeguarding. It might add just a little dust to the works, but that's always something I can deal with. |
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| dangus |
| Try running a good quality PC power supply fan on reduced voltage. Really old computer power supplies often have very high quality fans. If you run a 12V fan on 7 volts or so, they're a whole lot quieter, and still move enough air to make a big difference. I've got one running from an old cell phone charger that put out about 7 volts; see what you can find at a thrift store. |
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| bluebeard |
Dangus. We're thinking alike. I actually did that a couple of days ago. I just put a little 200ma 6.5vdc "wall wart" inside along with a 3.5inch sunon fan drawing air into the bottom vent. Was just the trick.
Thanks for all the input guys. I hope this thread can be left to sink into a semi-graceful oblivion now. It's gotten embarassingly long. |
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